Interferometric SAR systems on aircrafts have long been used to make height maps of the earth's surface. Typically the aircraft flies over a targeted region along a flight track to collect data. An antenna on the aircraft transmits electro-magnetic pulses, which are reflected off the target area and then received by the antenna using a signal receiver. The image data collected by each antenna is then used to create a two dimensional image of the targeted region from the perspective of the collecting antenna. Interferometric SAR technology takes advantage of the coherent interference of the two 2D images from the two antennas. In particular, SAR interferometry employs the phase difference resulting from the relatively small difference in the slant range from a point in the targeted region to the two antennas to extract data for a third dimensional parameter. The third parameter is preferably the elevation or height variation of the targeted area. This elevation information allows a 3-dimensional topographic map to be obtained.
In the aircraft version of a interferometric SAR system, physically separated antennas may be used bistatically with a single transmitter. Although the antenna separation is of necessity small, the interferometric baseline length relative to the imaging range is satisfactory.
In another type of SAR system, a pair of space vehicles, such as satellites, may be employed to collect SAR image data for interferometric processing. Each space vehicle is equipped with an antenna to collect reflected energy from the targeted region. With satellite IFSAR, the long imaging ranges from low-earth orbit dictate the need for substantial baselines (typically on the order of mile or so) to obtain good performance. With satellite IFSAR, obtaining sufficiently long baselines can be difficult because it is not practical to connect two space vehicles with booms or tethers of sufficient length, and with unconnected space vehicles, the relative movements of the antennas may be difficult to monitor and control.
It has further been observed that topographic information is not obtainable if the surface of the targeted area moves or changes between the two SAR image data collections causing decorrelation of the SAR image data. For example, wind can cause branches of trees and bushes to move so that no topographic data can be derived from that part of the scene as a result of the decorrelation. If the collections are separated too much in time, even normal vegetative growth can prevent the generation of topographic data.